1. Field of the Invention
The present invention relates to an image processing apparatus and image forming apparatus for processing both image data to express a character line image and half-tone density data.
2. Description of the Related Art
Anti-alias processing is well known which is a method for apparently smoothing zigzag edges when vector data to express a character line image is converted into raster data. Referring to FIGS. 50A to 50E, an example of the anti-alias processing is explained below.
First, in FIG. 50A represents a pixel which is an object to be processed. The size shown in the drawing is the same as the size of one pixel. This pixel is divided into a plurality of sub-pixels (16 sub-pixels in this drawing) as shown in FIG. 50B, and then vector data is put on the sub-pixels as shown in FIG. 50C. A value of the sub-pixel, upon the half portion and more of which the vector data is put, is defined as "1 (black)", and a value of the sub-pixel except for that is defined as "0 (white)" as shown in FIG. 50D. When a total of the values of 16 sub-pixels is defined as a pixel value of the pixel concerned, multi-value raster data can be generated in which the half-tone data is arranged in the edge portion. In this example, as shown in FIG. 50E, the pixel value is "5". Since the pixel is divided into 16 sub-pixels in the example shown in FIG. 50, gradation of the multi-value raster data is composed of 17 steps from "0" to "16". In this case, when the pixel is divided into more sub-pixels, it is possible to obtain multi-value raster data, the number of gradation steps of which is larger. Anti-alias processing described above has been improved in various ways, for example, the following are proposed to provide a higher printing quality.
Japanese Unexamined Patent Publication No. 4-195268 discloses the following technique. According to an inclination of vector data, a shape of the sub-pixel is changed, and a ratio of the area of the vector data in the pixel concerned is reflected in the multi-value raster data as accurately as possible, and this data is printed out by a multi-value laser printer. In this connection, FIG. 51 is a view showing a laser beam lighting signal control section of a laser printer capable of outputting multi-value raster data. As shown in FIG. 51, multi-value raster data S10 is converted into an analog signal S20 by a D/A converter, and the thus converted analog signal is compared with a reference triangular wave signal S30. The result of this comparison is used as a laser control signal. By this laser control signal, a laser beam is subjected to the pulse width modulation (PWM), so that a half-tone image can be outputted.
FIG. 52 is a view showing models of the signals S20, S30 and S40 of FIG. 51 when the above technology is applied to xerography (electrophotography) of the image writing system in which the laser is turned on in a period of time in which toner is made to adhere onto a sheet of paper. As shown in this drawing, the image signal S20, which has been converted into an analog value, is compared with the reference triangular wave S30, and the laser is turned on, that is, the laser is controlled to be lit when the image signal is higher than the reference triangular wave.
Japanese Unexamined Patent Publication No. 5-328108 discloses the following technique. An edge portion of a multi-value image data is detected, and the reference wave form is changed over in accordance with the result of the edge detection. In this way, the edge portion is smoothly outputted. For example, in the circuit shown in FIG. 53, a direction of the edge of the multi-value raster image is detected, and three types of triangular waves (shown in FIG. 54) are appropriately changed over in accordance with the result of the detection. In this way, it becomes possible to obtain a laser lighting signal shown in FIG. 55. When the laser lighting signal shown in FIG. 52 is compared with that shown in FIG. 55, the edge portion shown in FIG. 52 is separate from the center, however, the center of the edge portion is continuous in FIG. 55. That is, printing is conducted in a good condition in FIG. 55.
Japanese Unexamined Patent Publication No. 4-148949 discloses a vector image printer in which the operation is conducted as follows. While the anti-alias processing is being conducted, a vector image is developed in the image data and a predetermined image processing is conducted, and then the data is printed by the multi-value printer. This printer includes: an image data accommodating means for accommodating image data which has been subjected to the anti-alias processing; and a characteristic value accommodating means for accommodating characteristic information of the image data. Image data and characteristic information are simultaneously read out from these means while the characteristic information is referred to, and the corresponding image data is printed in the most appropriate condition.
In this connection, the following problems may be encountered in the anti-alias processing disclosed in Japanese Unexamined Patent Publication No. 4-195268. When the multi-value raster data is generated from the vector data, the sub-pixel shape is changed by an inclination of the vector data in the anti-alias processing, so that an area ratio of the vector data occupied in the pixel concerned can be reflected in the multi-value raster data as accurately as possible. However, the multi-value raster data, which has been generated, is provided with only the density information. Accordingly, when an image of small characters or an image containing lines is processed, a problem of "block" is caused.
The following problems may be encountered in the anti-alias processing disclosed in Japanese Unexamined Patent Publication No. 5-328108. Edge detection is conducted on the multi-value raster data. Accordingly, this anti-alias processing is effective when relatively large characters are processed. However, when small characters are processed, there is a possibility that the edge detection is not successfully conducted.
The following problems may be encountered in the anti-alias processing disclosed in Japanese Unexamined Patent Publication No. 4-148949. Although the above defects are not caused in this case since the control is conducted in accordance with the characteristic information of image data, it is necessary to provide a characteristic information accommodating means for all pixels in addition to the image data accommodating means. Accordingly, it becomes necessary to increase the memory capacity.